1) Field of the Invention
The present invention relates to a technology for detecting deterioration of the quality of an image when the image is written with a laser beam, controlling an image forming process based on detection and evaluation of graininess of a formed image, and controlling the image quality according to the detected deterioration.
2) Description of the Related Art
It is widely known that an amount of toner adhering to patch patterns can be detected by detecting reflected light quantity when a relatively large spotlight (a diameter of the spotlight is several millimeters or larger) is radiated onto the patch patterns formed on an image carrier. Furthermore, a method of controlling such image forming conditions as electrostatic latent image forming conditions and developing conditions based on a result of detection of the toner amount is also well known. This method is applied to actual products. If this detection method is used, by detecting toner adhesion quantity in each density patch of a gradation pattern, it is possible to get to know gradation characteristics and a solid density in the image forming conditions when the image is formed. Therefore, if values of the gradation characteristics and the solid density are beyond specified value ranges, the gradation characteristics and solid density can be changed in accordance with the detection result and by controlling the image forming conditions so s to obtain appropriate gradation characteristics and solid density.
Meanwhile, it is well known that the image quality has many factors such as the gradation characteristics, the solid density, and many other elements. Among the elements that influence the image quality greatly, “graininess” (a sense of roughness of the image that visually appeals to a human) can be pointed out. It has become essential to keep the graininess in a low level to realize a high quality image in an electrographic process. The graininess is largely determined by an initial image forming condition, however, in addition it is well known that the graininess deteriorates with time. Causes of the deterioration with time are attributed to environmental fluctuations such as fluctuations in temperature or humidity, or to deterioration of developer or photoreceptors. Therefore, it is necessary to detect the graininess or the image quality which is closely related to the graininess by adopting some measures in order to maintain the high quality image for a long period of time, and to change the image forming conditions based on results of the detection.
However, there have been no reports on measures to detect the image quality focusing on the graininess so far. The graininess is density unevenness on a plane space where the image is formed. In the case when human visual characteristics are taken into consideration,
making approximately 1 cycle/mm as a peak, the graininess is determined by the density unevenness having space frequencies in a range of
0 cycle/mm to approximately 10″ cycle/mm, especially,
making approximately 1 cycle/mm as a peak, particularly, the density unevenness having space frequencies in a range of
0.2 cycle/mm to approximately 4 cycle/mm becomes a problem.
Therefore, it is necessary to provide unit to detect the density unevenness in the range of the space frequencies mentioned above and unit to convert the detected density unevenness signals into a spatial frequency response.
On the other hand, as a unit to detect fine density unevenness in a patch pattern, an invention disclosed in Japanese Patent Application Laid-Open (“JPA”) No. H6-27776 is well known. The invention disclosed is provided to irradiate a wide range of the patch pattern with an illumination light to scan the light reflected from the patch pattern by a high-resolution charge coupled device (CCD), and to obtain signals related to fine image defects, based on the light reflected from the patch pattern. Further, even though the invention disclosed in JPA No.H6-27776 is provided with a process to compute space modulation transfer function (MTF) in a computing process, it is impossible to obtain information related to the space frequencies of image unevenness in this computation, consequently it is impossible to obtain information related to the graininess or information that has a strong correlation with the graininess. Further, in the invention disclosed here, the image forming condition is controlled based on a detection of an abnormal image such as lack of an image in the middle due to faulty transfer or on a detection of sharpness, but is not always controlled in consideration of the graininess.
Further, there are some other known inventions disclosed in JPA No. H5-161013, JPA No. H7-78027, JPA No. 2000-98708, or JPA No. 2001-78027. However, none of the inventions mentioned here controls the image forming conditions based on information for the graininess (density unevenness) of the image.
As explained above, in the conventional technology, the image forming conditions are not controlled in consideration of the graininess of toner, and therefore it is impossible to take countermeasures against deterioration of the graininess. In other words, the conventional technology is not provided to have such image quality detecting unit and image quality restoration unit, thus the developer or photoreceptors have to be inevitably replaced after reaching a certain operating hours estimated during a stage of development of the image forming apparatus. The replacement time had to be set shorter than an actually necessary time in consideration of a safety factor. In actual cases, however, running conditions of the image forming apparatuses differ from users to users, and therefore the replacement time of the developer and the photoreceptors that can guarantee the image quality should largely differ accordingly.
Furthermore, in the conventional technology, only settings and changes of the image forming conditions are proposed so that gradation characteristics (halftone density) and solid density become predetermined values. As mentioned above, the image forming conditions to be controlled have been developer toner concentration (in the case of a two-component developing process), a developing bias, and a developing roller speed. For example, if the image density is declined, no steps other than changes of optionally combined image forming conditions as shown below that are commonplace in electrographic process have been implemented:    Raising developer toner concentration    Raising developing bias (developing potential)    Raising linear velocity of developing rollers.